Re: [PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize direct map fragmentation

[Mike Rapoport <rppt@linux.ibm.com>]

On Tue, Sep 29, 2020 at 04:12:16PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 29, 2020 at 04:05:29PM +0300, Mike Rapoport wrote:
> > On Fri, Sep 25, 2020 at 09:41:25AM +0200, Peter Zijlstra wrote:
> > > On Thu, Sep 24, 2020 at 04:29:03PM +0300, Mike Rapoport wrote:
> > > > From: Mike Rapoport <rppt@linux.ibm.com>
> > > > 
> > > > Removing a PAGE_SIZE page from the direct map every time such page is
> > > > allocated for a secret memory mapping will cause severe fragmentation of
> > > > the direct map. This fragmentation can be reduced by using PMD-size pages
> > > > as a pool for small pages for secret memory mappings.
> > > > 
> > > > Add a gen_pool per secretmem inode and lazily populate this pool with
> > > > PMD-size pages.
> > > 
> > > What's the actual efficacy of this? Since the pmd is per inode, all I
> > > need is a lot of inodes and we're in business to destroy the directmap,
> > > no?
> > > 
> > > Afaict there's no privs needed to use this, all a process needs is to
> > > stay below the mlock limit, so a 'fork-bomb' that maps a single secret
> > > page will utterly destroy the direct map.
> > 
> > This indeed will cause 1G pages in the direct map to be split into 2M
> > chunks, but I disagree with 'destroy' term here. Citing the cover letter
> > of an earlier version of this series:
> 
> It will drop them down to 4k pages. Given enough inodes, and allocating
> only a single sekrit page per pmd, we'll shatter the directmap into 4k.

Why? Secretmem allocates PMD-size page per inode and uses it as a pool
of 4K pages for that inode. This way it ensures that
__kernel_map_pages() is always called on PMD boundaries.

-- 
Sincerely yours,
Mike.